1. Field of the Invention
This invention relates to a rotary drive unit for preventing the occurrence of vibration of a rotary shaft and more particularly to an improvement in a rotary drive unit or so-called "rotary actuator" which will be suitable for locating the head of a magnetic disk to a desired position of a disk surface.
2. Description of the Prior Art
The following techniques are known as the prior art techniques for the rotary actuator for locating the head of a magnetic disk.
The first of the prior art techniques is described in "IBM Journal of Research and Development", July, 1976, pp. 389-397. According to this reference, a carriage equipped with the head at one of its end portions is fitted to the rotary shaft and a coil of a voice coil motor is fitted to the opposite side of the carriage with the rotary shaft as the center or in other words, to the other end portion of the carriage. Since magneto-electric interaction occurs between the coil of the voice coil motor and a magnetic circuit, the coil is driven around the axis of the rotary shaft together with the carriage and with the rotary shaft and the head can thus be located to a desired position of the disk.
The second of the prior art techniques is described in "IEEE, TRANS. MAGNETICS", MAG17 (1981), pp. 1387-1395. In this reference, a carriage having a head at one its end portions is fitted to a rotary shaft and the coil of a voice coil motor is directly fitted to this rotary shaft. The coil consists of two coils that are disposed at mutually symmetric positions on the outer side of the rotary shaft and one of the coils is disposed at one of the end portions of the carriage with the other being disposed at the other end of the carriage.
In the rotary actuator for use in the magnetic disk, the head moves and is located with an accuracy of a micron order on the disk. Unless precise balance is established around the rotary shaft, therefore, vertical force acts on the rotary shaft and vibration is likely to occur. It is therefore necessary to carefully consider the arrangement wherein the weight on the load side such as the head is in equilibrium with the weight on the opposite side with respect to the rotary shaft when dimensions of constituent components are determined.
An aluminum wire having a smaller specific gravity than a copper wire is generally used for the coil of the voice coil motor in order to improve response of the head.
In accordance with the first prior art technique described above, the coil of the voice coil motor is fitted to the other end portion of the carriage which is equipped with the head at one of its ends. However, since the coil is much lighter than the load such as the head, a balance weight is fitted to the other end of the carriage to establish the balance. If a copper wire having a greater specific gravity is used for the coil in this case, the coil is by far heavier than the load. Though a balance weight may be fitted to one of the ends of the carriage to establish the balance, the total weight of the apparatus becomes further greater and response of the head will drop undesirably.
In accordance with the second prior art technique described above, the coil consists of two coils that are disposed symmetrically. Therefore, though the coil balances itself but since one of the coils is disposed on the same side as the head, the other coil side is lighter so that the balance weight must be fitted to the other end of the carriage in the same way as in the first prior art technique.
However, the first and second prior art techniques involve the drawbacks in that they require more complicated assembly line because they use the balance weight, an extra space is necessary for the balance weight and since vibration resulting from the balance weight occurs, total vibration cannot be reduced.